Typically, a solid oxide fuel cell (SOFC) employs a solid electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (hereinafter also referred to as MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
Normally, a hydrogen gas produced from hydrocarbon based raw fuel by a reformer is used as a fuel gas supplied to the fuel cell. In the reformer, in general, a reformed gas (fuel gas) is produced, e.g., by applying partial oxidation reforming or steam reforming to such hydrocarbon based raw fuel, e.g., fossil fuel such as metal or LNG.
In this case, since the partial oxidation reformer induces exothermic reaction, the reaction can be started at relatively low temperature, and thus the start-up performance and the follow up performance are good. However, the reforming efficiency is poor.
In contrast, the steam reformer has good reforming efficiency. However, since the steam reformer induces endothermic reaction, the start-up performance and the follow up performance are poor at relatively low temperature.
In this regard, for example, a starting-up method as disclosed in Japanese Laid-Open Patent Publication No. 2006-190605 (hereinafter referred to as Conventional Technique 1) is known. As shown in FIG. 9, Conventional Technique 1 relates to a method for starting-up a SOFC system equipped with a reformer 1a having a reforming catalyst, and an SOFC 2a which uses the reformed gas as a fuel.
The reformer 1a is provided in a manner that a reforming reaction tube 3a containing the reforming catalyst is disposed in a vessel 4a or runs through the vessel 4a. In the reforming reaction tube 3a, an upstream part thereof is filled with a partial oxidation reforming catalyst A and a downstream part thereof is filled with a steam reforming catalyst B, to form a reforming catalyst layered structure.
In the starting-up method, catalyst A having a partial oxidation reforming (POX) function and catalyst B having a steam reforming (SR) function are used. The method includes the steps of: increasing the temperature of catalyst A, by combustion heat or electricity, to a temperature at which POX reaction can proceed; increasing the temperature of catalyst B by POX reaction heat, increasing the temperature of SOFC 2a by feeding the reformed gas to an anode and heating catalyst B by combustion heat generated from combustion of a reformed gas discharged from the anode, or increasing the temperature of catalyst B by POX reaction heat, increasing the temperature of SOFC 2a by feeding a combustion gas produced from combustion of the reformed gas to a cathode and heating catalyst B by this combustion gas; and after catalyst B has been heated to a temperature at which SR reaction can proceed, reducing the proportion of POX reaction or stopping POX reaction and perform SR.
Further, as shown in FIG. 10, a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2005-293951 (hereinafter referred to as Conventional Technique 2) includes a body 1b of a solid oxide fuel cell, a first fuel gas supply system 2b for supplying a fuel gas to the anode side of the fuel cell body 1b at the time of power generation, a second fuel gas supply system 3b for supplying a small amount of the fuel gas as an oxidation-prevention gas to the anode side of the fuel cell body 1b during a period of starting operation of the fuel cell body 1b and a period in which operation of the fuel cell body 1b is stopped, and an oxygen-containing gas supply system 4b for supplying air as an oxygen-containing gas to the cathode side of the fuel cell body 1b all the time.
The fuel cell body 1b includes a fuel cell FC having a columnar shape formed by stacking a plurality of inter-connectors such that solid electrolytes with electrodes are positioned between the inter-connectors, a burner 5b for pre-heating the fuel cell FC during the period of starting operation, a heat exchanger 6b that is pre-heated together with the fuel cell FC during the period of starting operation and pre-heating the oxygen-containing gas by heat exchange of the exhaust gas discharged from the fuel cell FC with the oxygen-containing gas, and a steam reformer 7b and a partial oxidation reformer 8b as two types reformers. The steam reformer 7b and the partial oxidation reformer 8b are used in combination with the fuel cell FC, and pre-heated together with the fuel cell FC by the burner 5b or the like in the period of starting operation, and heated by heat produced in the fuel cell FC during power generation.
During power generation, the fuel gas required for power generation is supplied to the fuel cell body 1b at a large flow rate using the first fuel gas supply system 2b, and during the period of starting power generation and during the period where power generation is stopped, when the fuel cell body 1b goes through a cell oxidation temperature range, the fuel gas produced by partial oxidation reforming is supplied to the fuel cell body 1b at a small flow rate which is required for prevention of oxidation, by use of the second fuel gas supply system 3b. 